This invention relates generally to a method of controlling the spread of an adhesive on a circuitized organic substrate. More particularly, this invention relates to a method of forming a protective coating on the surfaces of a circuitized organic substrate. Such protective coating reduces the spread of adhesives that contact or are disposed on the surfaces of the substrate.
Semiconductor devices typically comprise an integrated circuit die and a plastic chip carrier that is comprised of an organic substrate, such as polyimide, having metallic circuitry and wire bond pads disposed on a surface thereof Frequently, one of the initial stages in assembly of the semiconductor device involves attachment of the chip carrier to a lead frame by means of a thixotropic organic adhesive such as, for example, an epoxy-based adhesive, an acrylic-based adhesive, or a silicone. During this process, which is referred to as the board-attach process, the adhesive is applied to the lead frame. Then the chip carrier is placed onto the adhesive. The assembly may then be heated to assist in cure of the adhesive, thereby strengthening the attachment between the organic substrate and the lead frame.
Adhesives are also used to attach the integrated circuit die to the circuitized surface of the chip carrier. Such adhesives, generally, contain an organic binder and an electrically or thermally conductive filler. The adhesive, which initially is in the form of a viscous liquid or paste, is distributed onto preselected regions on the circuitized surface. The preselected regions include chip pads which are formed by selectively plating gold on a nickel plated copper surface. Thereafter, the perimeter of integrated circuit die is aligned with and placed on the adhesive. The entire structure, including substrate, adhesive and die, is then baked to cure the adhesive, thereby strengthening the attachment between the integrated circuit die and the underlying substrate. In some cases, the integrated circuit die is then electrically connected to the electric circuitry of the carrier by wire bonding terminals on the chip to wire bond pads on the surface of the substrate.
Although the adhesives used during the various stages of assembling the resulting module are fairly viscous, the organic liquids in the adhesives have a propensity to bleed and spread out away from the point of attachment. For example, during the board attach process, these organic liquids often bleed out from the periphery of the chip carrier attachment site and spread up the edges of the chip carrier onto the circuitized upper surface, where the organic liquids can contaminate the wire bond pads and render them non-bondable. This condition, which is hereinafter referred to as xe2x80x9cadhesive resin bleedxe2x80x9d, can cause significant problems during later assembly steps when the bond sites are needed to complete necessary electrical connections. The spreading organic liquid can also contaminate any portions of a soldermask which may lie in the near vicinity of the wire bond pads.
Adhesive resin bleed is also encountered during other stages of assembling the resulting module. For example, the resin in the electrically conductive or non-electrically conductive adhesive that is used to attach the integrated circuit die to the chip carrier can also bleed out during cure. The heat and pressure encountered during cure causes the resin to bleed out from the periphery of the die attachment area and spread out over adjacent areas where electrical connections ultimately need to be made.
The problems associated with adhesive resin bleed are even more pronounced when the chip carrier is treated with a plasma containing oxygen and/or argon, prior to the application of the adhesive. Such plasma treatment is frequently employed prior to assembly of the semiconductor module to clean the wire bond pads and to roughen the surface of the substrate. Adhesive resin bleed becomes an even greater problem when the surface of the substrate is roughened by pumice treatment.
Various methods for reducing contamination of the electrical bonding sites by adhesive resin bleed have been developed. For example, the chip carrier surface may have a recess at the point of attachment of the chip, such that the back of the chip and the adhesive will be positioned below the adjoining circuitized surface of the chip carrier where electrical bonding sites are located. Unfortunately, not all integrated circuit assemblies provide the option of a recessed cavity in the carrier surface. Very large scale integrated assemblies require a large number of bonding sites, and these sites, typically, are on the same level of the carrier surface as the chip attachment site.
In some instances mechanical barriers have been used to control adhesive resin bleed onto the bonding sites. For example, in U.S. Pat. No. 5,409,863 issued on Apr. 25, 1995, a low profile barrier, such as a solder mask ring surrounds the chip attachment site. During cure, the ring prevents the spread of adhesive resin onto the adjacent bonding sites on the chip carrier. Unfortunately, the ring does not completely prevent adhesive resin bleed when the chip carrier is treated with an oxygen or argon containing plasma.
Accordingly, a chip carrier having a surface that is resistant to adhesive resin bleed is desirable. Furthermore, a method of imparting adhesive resin repellency to the surface of the chip carrier and to the metallic components on the surface is desirable. A method which does not reduce the bondability of the wire bonding sites is especially desirable.
In accordance with the present invention, a method of treating the surface of an organic substrate, particularly a circuitized surface of a chip carrier, which method reduces the spread of adhesive resin that is subsequently deposited on the surface, is provided. This method comprises the steps of applying a treatment solution comprising a fatty acid compound, an alkalizing agent, and a solvent comprising water and from about 5% by volume to about 90% by volume of an organic solvent selected from the group consisting of alcohol, glycol ether, and combinations thereof to the surface; and then removing substantially all of the solvent from the solution to provide a thin film on the surface of said substrate. The film comprises the fatty acids that were present in the treatment solution. In a preferred embodiment the treatment solution further comprises a chelating agent.
The present invention also relates to a treatment solution for reducing adhesive resin bleed on the surface of a circuitized organic substrate. The present invention also relates to a circuitized organic substrate that is resistant to resin bleed. Such substrate has a film comprising a fatty acid compound disposed on the circuitized surface thereto. In accordance with the present invention, it has been determined that organic substrates having a film comprising a fatty acid compound on the surface thereof are less wettable with respect to the organic liquids that are normally present in the adhesives that are used to bond chip carriers to integrated circuit chips and to lead frames. It has also been determined that the presence of the film on the surface of the organic substrate reduces adhesive resin bleed. It has also been determined that the presence of the film on the metallic components of the circuitized surface of a chip carrier renders the metallic components less wettable to the organic liquids present in such adhesives. It has also been determined that the presence of the film does not adversely affect the subsequent bonding of wires to the metal components on the surface of a chip carrier.
In accordance with the present invention, a method for conditioning a surface of an organic substrate, particularly a circuitized surface of an organic substrate, to provide a surface that is resistant to adhesive resin bleed is provided. The method comprises the steps of applying a treatment solution to the surfaces of the substrate and, preferably, to the surfaces of the metallic components on the circuitized surface of the substrate, and then removing substantially all of the solvent from the solution to provide a thin, dry, protective film on the treated surfaces of the substrate and metallic components. Such film makes the surface of the substrate less wettable with respect to the organic liquids that are found in the uncured adhesives that are used to attach chip carriers to lead frames and to integrated circuit dies. In the preferred embodiment, the substrate is a chip carrier.
Treatment Solution
The treatment solution has a pH of from about 7.2 to about 10.6 and comprises a fatty acid compound, an alkaling agent, an aqueous or water-based solution comprising from about 5% to about 70% of an organic solvent selected from the group consisting of alcohol, glycol ether, and combinations of such solvents and, preferably, a chelating agent. Fatty acid compound as used herein means a fatty acid, a salt of a fatty acid or an ester of a fatty acid. Fatty acid compounds suitable for forming the treatment solution can be represented by the following formula: 
wherein R1 is a saturated or unsaturated hydrocarbon having 5 to 32 carbon atoms;
Y1 is a H, a hydrocarbon, or a hydroxyl at any position along R1;
Y2 is a H, a hydrocarbon, or a hydroxyl at any position along R1 
Z is O, N, or S; and
R2 is a saturated or unsaturated, branched or unbranched hydrocarbon having 1 to 12 carbon atom, a salt, or H.
The concentration of the fatty acid compound in the treatment solution is from about 0.001% to about 0.5% by weight of the solution, preferably from about 0.001% to 0.01% by weight of the solution.
Suitable alkalizing agents include but are not limited to KOK NaOH, Ca(OH)2, NH4OH, and triethanolamine. Preferably the alkalizing agent comprises less than 5% by weight of the treatment solution. Chelating agents as used herein refer to compounds which are used to remove metal ions from a solution. Suitable chelating agents include, but are not limited to, EGTA, triethanolamine, and, preferably, EDTA Preferably, the chelating agent comprises from 0.01 to about 3% by weight of the treatment solution.
Alternatively, a concentrated solution of a product from ENTHONE OMI, Inc., New Haven, Conn., available under the trade name ENTEK 996 can be combined with a solvent which contains water and, preferably, an alcohol, to prepare the treatment solution. ENTEK 996 comprises a fatty acid, EDTA, butyl carbitol, sodium hydroxide, and water. In instances where ENTEK 996 is used to prepare the treatment solution, it is preferred that ENTEK 996 comprises from about 0.05 to about 10% by volume, more preferably from about 0.2 to about 5% by volume, most preferably from about 0.9% to about 3% by volume of the final treatment solution.
Suitable solvents include but are not limited to aqueous or water-based solutions that contain water, preferably deionized water and from about 5% to about 90% by volume of an organic solvent selected from the group consisting of an alcohol, a glycol ether, and combinations thereof Preferably, the aqueous solution comprises from about 5% to about 70% by volume of the organic solvent, more preferably from about 10% to about 30%. The preferred alcohol is an aliphatic alcohol. Representative alcohols are, by way of example, methanol, ethanol, propanol, isopropanol, butanol, pentanol, and amyl alcohol. Suitable gycol ethers are, for example, diethylene glycol monobutyl ether, ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, diethylene glycol monomethyl ether acetate or ethylene glycol monometh ether acetate. Most preferably, the solvent is an aqueous solution comprising an alcohol, wherein the alcohol is from about 10% to about 30% by volume of the aqueous solution.
The treatment solution is heated to a temperature of from about 60xc2x0 C. to about 80xc2x0 C. and then applied to the surface of the substrate. The solution is applied by dipping the substrate in the solution, flowing the solution over the surface of the substrate, or spraying the surface of the substrate with the solution, preferably using a fluid-head apparatus. The process time for treatment with the heated treatment solution is preferably from about 30 seconds to about 60 seconds.
The treatment solution can be applied to any organic resinous material which normally is used to form the underlying substrate of a semiconductor chip carrier. Examples of resinous materials which are used to form such underlying substrates include for example epoxy-based resins, epoxy resins reinforced with woven fiberglass, polyimides and bismaleimide/triazine resins and combinations thereof. Furthermore, the treatment solution may also be applied to the inorganic materials that are used to form the circuit lines, the ground pads, and the wire bond sites which are on the surface of the circuitized substrate. Such inorganic materials include, for example, copper from which the circuitry is formed and gold, which along with nickel is plated over the copper circuitry to provide a wire bondable surface and to provide protection against corrosion
Following application of the treatment solution to the desired surfaces, substantially all of the solvent is removed from the applied solution to provide a thin film of fatty acids on the surface of the substrate. Solvent is removed from the treatment solution at a time and temperature effective to remove at least 97%, preferably greater than 99% of the solvent from the applied solution. The solvent may be removed by heating the surface of the substrate, baking the substrate or, preferably, by air drying the substrate at room temperature. It has been determined that baking the substrate at a temperature of from about 120xc2x0 to 140xc2x0 C. for about 1 to 10 minutes achieves the desired result. Removal of the solvent results in the disposition of a film comprising the respective fatty acid compound on the surface to which the solution has been applied. The film comprises from about one to about 3 layers and has a thickness of less than 300 angstroms, preferably less than 100 angstroms, more preferably of from about 40 to about 60 angstroms.
Following formation of the film, an adhesive, preferably a bonding adhesive is applied to a lead frame. The substrate is then placed on a lead frame, and the adhesive cured under conditions of elevated temperature for a time sufficient to attach the substrate to a lead frame. Then an adhesive, preferably a silver containing die attach adhesive is applied to selected chip bonding sites on the top surface of the substrate. Thereafter, an integrated circuit die is placed on the adhesive, which is then cured under conditions of elevated temperature and pressure to bond the integrated circuit die.
It has been found that treatment of a circuitized organic substrate in accordance with the present invention renders the surface of the organic substrate and the surface of any metallic components that, preferably, are present on a surface of the substrate less wettable to the organic liquids contained in a typical uncured adhesive material, such as, for example, a liquid epoxy. It has also been found that treatment of the circuitized organic substrate in accordance with the present invention minimizes or even eliminates resin bleed from adhesives that are subsequently deposited onto or that come into contact with a surface of the substrate, even when such substrates have been subjected to pumice treatment. As a result, contamination of the wire bond pads by the organic liquids that are present in the uncured adhesive is eliminated when chip carriers are treated in accordance with the present invention.
It has also been found that treatment of the circuitized substrate in accordance with the present invention results in the formation of a thin fatty acid compound-containing film on the surface of the organic substrate, the surface of the metallic components plated thereon, and on the surface of any solder masks which are present on the surface of the substrate. It has also been found that the presence of such film on the surface of the metallic components does not diminish the strength of the bonds formed between the wire bond sites on the surface of the substrate and the wires which are subsequently used to connect the wire bond sites to the integrated circuit die nor diminishes the integrity of this electrical contact.